1. Field of the Invention
This application is related to communications systems and more particularly to receivers of communications systems.
2. Description of the Related Art
A typical superheterodyne receiver converts a received radio-frequency (RF) signal to an intermediate-frequency (IF) signal. The receiver demodulates the IF signal using synchronous detection driven by a local oscillator having a frequency that is substantially the same as the frequency of the carrier signal for the intended data signal. A typical direct-conversion (i.e., homodyne, synchrodyne, or zero-IF) receiver has a local oscillator frequency that is approximately the same as the frequency of the carrier signal. Mixing in a direct-conversion receiver converts the received RF signal directly to baseband (i.e., zero frequency). That is, a direct-conversion receiver converts a received RF signal to a baseband signal using a single frequency conversion. The typical direct-conversion receiver is less complex than the typical superheterodyne receiver since the direct-conversion receiver requires fewer frequency conversions, eliminates intermediate frequency stages, and reduces image rejection issues. The reduced complexity of direct-conversion receivers results in compact digital signal processing code size, efficient digital signal processing data manipulation, and reduced integrated circuit area.
However, a typical direct-conversion receiver introduces a DC offset into the received signal due to RF and static DC mechanisms. Static DC mechanisms include DC offsets introduced in the mixer and/or amplifiers due to mismatches in device layouts and manufacture. Radio-frequency mechanisms include self-mixing due to pickup of the receiver local oscillator signal at an input of a low-noise amplifier (LNA) and at a frequency mixer input. For example, local oscillator energy may leak through the frequency mixer, feed back to the receiver antenna input, and then re-enter the frequency mixer. As a result, the overall local oscillator energy self-mixes and creates a receiver DC offset signal. The receiver DC offset signal may be large enough to overload any baseband amplifiers and degrade the recovery of an intended data signal. Typical receiver modifications that may reduce the receiver DC offset include high-pass filtering the received signal, which may reduce the realizable throughput of the receiver and increase the complexity of the receiver. The increased complexity is associated with higher production costs that may outweigh the benefits. Accordingly, improved techniques for recovering an intended data signal from an RF signal in a direct-conversion receiver are desired.